Optical inspection of surfaces open to different directions in a piece of material

ABSTRACT

The invention relates to a device for optical inspection of the open surfaces ( 19  of objects from at least two different viewing directions (P 1 , P 2 ). The device comprises a telecentric imaging unit ( 11  or  12 , an angle mirror ( 13 ) and auxiliary mirrors ( 8 ) within the area (K) of the telecentric imaging unit, between this and the object. The object is placed between the arms ( 3   a   , 3   b ) of the angle mirror ( 13 ) and the telecentric imaging unit is directed towards the combination of object and angle mirror. The auxiliary mirrors ( 8 ) have been oriented and positioned at intervals from the telecentric imaging unit such that the differences of distance of the viewing directions (P 1  and/or P 2  and/or P 3  and/or P 4 ) via the two arms ( 3   a  and  3   b ) of the angle mirror or via one arm ( 3   a  tai  3   b ) or not via the arms are compensated as they pass via the auxiliary mirrors.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is for entry into the U.S. national phase under §371for International Application No. PCT/FI05/000182 having aninternational filing date of Apr. 12, 2005, and from which priority isclaimed under all applicable sections of Title 35 of the United StatesCode including, but not limited to, Sections 120, 363 and 365(c) andwhich in turn claims Priority under Section 119 to Finnish PatentApplication No. 20040694 which was filed on May 18, 2004.

TECHNICAL FIELD

The invention relates to a device for optical inspection of opensurfaces of objects from at least two different viewing directions, thedevice comprising: illuminating means (light or radiation source) forilluminating the open surfaces of the objects; sensor means (sensor) fordetecting the light intensity reflected by different locations of theopen surfaces of the objects and for converting the light intoelectronic form.

BACKGROUND OF THE INVENTION

A typical optical inspection apparatus consists of a radiation sourceand a camera composed of an objective and an image plane. The imagegenerated by the objective on the image plane can be inspected andstored e.g. by means of a CCD cell or a CMOS array, which converts theimage into an electric signal in a known manner. Such a cell consists oflight-sensitive elements disposed as a matrix, e.g. 256×256 elements orpixels. In this case, the properties of the camera resemble those of anordinary photographing camera. The image plane formed of pixels may alsohave e.g. the shape 1024×1, and this is then a “linear camera”. Thistext refers to a camera of the type described above as an electroniccamera and it can be used for taking moving pictures or, if desired,also still pictures—in other connections their established name is“video camera” or “digital camera”. Conventional camera optics views theobject in different ways depending on the location of the object in themeasurement area. At the optical axis of the objective, i.e. the centralarea of the object, the camera views the object at right angles, and atits edges at an oblique angle, which is larger the greater the distancefrom the optical axis. This is called the central perspective, whichcauses detrimental imaging errors for the measurement of the object andquality control in general, and these errors can be corrected by meansof telecentric optics. In that case, all the beams from the objectarrive in parallel with the optical axis and all the locations of theobject are viewed in the same plane perspective. In telecentricobjectives, the lens or concave mirror closest to the object should havea width equalling at least the object, and this results in heavy andbulky optical equipment consisting of ordinary lenses and/or mirrors. EP1 089 106 discloses a relatively light and simple solution to theseproblems. The telecentric design of this reference uses a strip-likeplanar parabolic mirror, the aperture of the objective proper beinglocated in the area of the focal plane of the mirror. This objectiveproper, in turn, is integrated in a non-telecentric camera, whichgenerates an image on a light-sensitive image plane.

In many cases, it is necessary to measure and/or inspect the object alsofrom other directions than one specific direction. Thus, for instance,it may be necessary to monitor sawn timber from the direction of twoopposite faces or from the directions of all four faces. This canobviously be done by means of four devices directed towards the uppersurface, lower surface and lateral surfaces of the object, but thisincurs high equipment costs. Another option is turning the object andrunning it through the imaging area of a telecentric imaging unit fourtimes. Firstly, such an arrangement is slow in terms of production andsecondly, mutual positioning of the image data obtained on differentsides of the object is problematic. WO 94/24516 depicts an arrangementfor measuring the width of a moving object by using two parabolicmirrors in connection with one camera, together with an elongated lightsource providing background light. For measuring the thickness of theobject, laser included in the arrangement and a second camera are used,and if necessary, a second laser and a third camera. This arrangementonly allows for measurement of the boundary dimensions of the object indifferent projections.

DE 41 04 501, again, explains an arrangement for determining the sapwoodside and the heartwood side of two even surfaces of sawn timber, such asplanks and boards. The reference makes a difference between theseopposite sides by utilising their different grain densities, i.e. growthring densities. In order to determine these different grain densities,the reference suggests passing the timber body between at least one pairof sensors, with the sensors disposed opposite each other. These atleast two sensors consist of a transmitter and a receiver operating inthe range of visible light or infrared light. The sensors identify thegrowth rings on the basis of the different light reflectivity ofadjacent locations on the timber body, and these initial data of theopposite sensors regarding differences in reflection density are fedinto a comparator in the reference, the comparator calculating by meansof not represented software which of the two opposite sides is sapwoodand heartwood, respectively. The inherent structure of the sensors hasnot been described in any way, however, the figures of the reference andthe definition “transceiver” allow the conclusion that the sensordetects only one pixel at a time, without any optics proper. Thereference does not specify whether the measurement is based on averagereflection densities of larger areas of the object—in the case of alarge-sized pixel—or on the reflection density provided by transverselymovable sensors or a plurality of sensors—in the case of a small-sizedpixel. In other words, the arrangement of this reference allowsobservation of two surfaces of the object, however, the number ofsensors is at least equal to the number of inspected surfaces. Thereference does not indicate the manner of inspecting the entire area ofeven one surface of the object.

SUMMARY OF THE INVENTION

Hence the invention has the purpose of providing an arrangement allowinginspection of the surface of an object in all its cross-sectionaldirections by optical means, i.e. an electronic camera. This means thatthe invention enables imaging and inspection of the upper surface, lowersurface and the lateral surfaces of the object, in other words, theobject is then usually viewed from four mutually perpendiculardirections, which are typically in the same plane, but at least inmutually parallel planes. Depending on the shape of the object, it is,of course, permissible to optionally image and inspect it from onlythree corresponding directions. The second purpose of the invention isto provide an arrangement allowing the imaging inspection of the typedescribed above to be carried out with a minimum number of cameras inorder to keep the expenditures low. The third purpose of the inventionis to provide an arrangement allowing the imaging inspection of the typeabove to be performed at surfaces of a movable object open to differentdirections so that all the surfaces of say, elongated bodies, are imagedand inspected if desired. The fourth purpose of the invention is toprovide an arrangement allowing the imaging inspection to be applied tothe analysis of all types of errors and properties of the object. Thismeans that the arrangement proper should not restrict the features ofthe object to be determined. The invention has the further purpose ofproviding such an arrangement allowing imaging inspection to be carriedout without repetition and using an apparatus devoid of moving parts.

The problems explained above are resolved and the purposes defined aboveare achieved with the arrangement in accordance with the invention.

One of the chief advantages of the invention is that the arrangement ofthe invention allows imaging of all the other surfaces of a movingobject, except the ends perpendicular to the direction of movement,using an electronic camera with a view to inspection and potentialmeasurement of the surfaces. The invention also has the advantage ofallowing such imaging using one single electronic camera, although twocameras can be used if desired, while the entire object is imaged bypassing it once through an apparatus corresponding to the arrangement ofthe invention, given the substantially simultaneous imaging of all ofthe outer surfaces of the object by means of the invention. Theinvention has the further advantage of maintaining all the outersurfaces simultaneously within the depth of field of the electroniccamera and also of eliminating the central perspective and replacing itwith true planar projections. In addition, the apparatus implementingthe arrangement of the invention has a compact and robust design andthus ensures excellent operational reliability.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in further detail below with reference to theaccompanying drawings.

FIGS. 1 and 2 are schematic views of a preferred embodiment of thedevice of the invention for inspecting the open surfaces of an objectfrom four viewing directions by using one single telecentric imagingunit comprising a video camera.

FIG. 1 shows the device in a direction perpendicular to the direction ofmovement of a moving object, corresponding to direction I in FIG. 2, and

FIG. 2 shows it accordingly in the direction of movement of the object,corresponding to direction II of FIG. 1.

FIGS. 3A-6A and 3B-6B illustrate separately the proceeding light beamsin four viewing directions, with the directions perpendicular to eachother.

FIGS. 3A, 3B show an arrangement for inspecting the lower surface of anobject,

FIGS. 4A, 4B an arrangement for inspecting the upper surface of anobject,

FIGS. 5A, 5B show an arrangement for inspecting a lateral side of anobject and

FIGS. 6A, 6B an arrangement for inspecting the other lateral side of theobject.

FIG. 7 is a schematic view of another embodiment of the device of theinvention for inspecting the open surfaces of an object from fourdirections using one single telecentric imaging unit comprising asurface camera in the direction of movement of a moving object, i.e. inthe same projection as in FIG. 2.

FIG. 8 is a schematic view of a third embodiment of the device of theinvention for inspecting the open surfaces of an object from fourdirections using one single telecentric imaging unit, in the directionof movement of the object, i.e. in the same projection as in FIGS. 2 and7.

FIG. 9 is a schematic view of a fourth embodiment of the device of theinvention for inspecting the open surfaces of an object from fourdirections using two telecentric imaging units, in the direction ofmovement of the object, i.e. in the same projection as in FIGS. 2, 7 and8.

FIGS. 10 and 11 illustrate the principle of the image plane of thetelecentric camera used in the invention including the light-sensitivepixels, in the case of a linear camera and a surface camera,respectively.

DETAILED DESCRIPTION

The figures illustrate a device for optical inspection of the opensurfaces 1 of the profiles of objects 10, especially objects moving inthe longitudinal direction, such as sawn goods or planed timber, butalso objects of other materials, from at least two different viewingdirections P1, P2. In this context, open surfaces 1 imply a surfacevisible from the outside and thus possible to inspect, but not e.g. theinside of pipes or hollow profiles, i.e. cavities. The open surfaces aregenerally referred to with reference numeral 1, which implies any opensurface of an object, the specifying reference numerals implying thelateral surfaces 1 c and 1 d and the lower surface 1 b and the uppersurface 1 a only in cases where it is necessary to make a differencebetween these. The objects move through the device of the invention inthe direction of movement D_(L). The device of the invention comprisesprimarily the illuminating means 20 for illuminating the open surfacesof the object. These illuminating means 20 may consist of any lampsemitting an adequate amount of light in a suitable wavelength range,possibly sources of diffused light or sources of directional light,which, however, emit incoherent radiation. Consequently, there are noother requirements on the light sources than those imposed on goodgeneral lighting, and they may be based on incandescent lamp techniques,fluorescence tube techniques, LED techniques (Light Emitting Diode) orany other similar, previously known or new techniques. Inspection of theopen surfaces 1 of the objects 10 naturally requires the use of one ormore specific illuminating wavelengths, which are, of course, used, thewavelength band of the illuminating radiation being broader or narrower,within the visible wavelength range, the infrared range, the ultravioletrange or the like. In this context, it is essential that there are norequirements regarding the parallel direction or coherence of theilluminating radiation, and hence there is no need for the use of one ormore lasers in connection with the invention.

In addition, the device of the invention comprises sensor means 30 fordetecting and converting into electric form the light intensityreflected from the different locations on the open surfaces 1 of theobjects. In a preferred embodiment of the invention, the sensor means 30consist of a first telecentric imaging unit 11, which consists of anon-telecentric camera 21 having an objective 22 and an image plane 23formed of light-sensitive pixels and having an optical axis 9, and of aconcave planar parabolic mirror 24 or possibly of a parabolic mirror inthe focal plane F of which there is an aperture 25. The telecentricimaging unit 11 has an imaging area K in at least one plane. The imagingarea K is formed by the joint effect of the image angle of the cameraand a concave mirror—or a lens, respectively—located between this andthe object, given the operation of the imaging unit with parallel beamsoutside the unit. The imaging area K is substantially in one plane whena linear camera is used, the type of image plane of this cameraappearing in FIG. 10, however, there may also be provided an imagingarea K in two different directions when a surface camera is used whoseimage plane type is shown in FIG. 11. Both of these have a real surfacearea, but a linear camera utilises primarily one dimension, whereas asurface camera utilises all the dimensions of the image plane. In thelatter case as well, the imaging areas in different plane directionsthrough different optical axes may be of different sizes, yet they areall covered by the name imaging area K in this description. Telecentricimaging units are commonly known and are thus not described in detailhere. A telecentric imaging unit that is especially suitable for use inconjunction with this invention has been explained in the applicant'sprevious publication EP 1 089 106. In connection with the inventionexplained here, a telecentric imaging unit is preferably used, by meansof which a non-telecentric camera is imparted telecentric properties byusing a concave mirror, especially a concave strip mirror, and mostadvantageously a planar parabolic strip mirror, as has been explained inthe reference mentioned above. It should be understood that there are noobstacles in terms of imaging theory to the use of a telecentric imagingunit of some other type, but such units can be used as well, providedthat problems relating to costs and usability can be resolved. If asecond telecentric imaging unit 12 is also used in this device, thissecond imaging unit 12 is of the same type as the first imaging unit 11and thus comprises a non-telecentric camera 21, which consists of anobjective 22 and an image plane 23 formed of light-sensitive pixels, anda concave planar parabolic mirror 24, with an objective aperture 25 inits focal plane F. Consequently, the telecentric imaging unit defined inthis description relates both to the first imaging unit 11 and thesecond imaging unit 12.

In accordance with the invention, the device further comprises an anglemirror 13, which consists of two arms 3 a and 3 b, which are planarmirrors, preferably planar strip mirrors, as shown in FIGS. 1 and 3A-6A.The angle bisector 14 of the angle mirror 13 and the optical axis 9 ofthe telecentric imaging unit 11, 12, or the angle bisector 14 and theoptical axes 9 of the telecentric imaging units 11, 12, respectively,are parallel, this definition comprising also the extensions of theangle bisector and the optical axis, as illustrated in FIGS. 2, 3B-6Band 7-9. The extensions are obvious, since turning the system into aphysically different position by means of mirrors changes all the otherfeatures accordingly, and then the same conditions still are valid. Inthe most typical case, the angle bisector 14 of the angle mirror 13 andthe optical axis 9 of the telecentric imaging unit 11, 12 or theirextensions join, as can be seen in FIGS. 2, 3B-6B, 7 and 8. The deviceof the invention further comprises at least one first auxiliary mirror 8a within the imaging area K of the telecentric imaging unit and inaddition, the object 10 is placed between the arms 3 a, 3 b of the anglemirror 13. If it is desirable to image and inspect all the open surfaces1 of the object 10, especially the side surfaces 1 c, 1 d that have adimension parallel with the optical axis 9 and the angle bisector 14,the object should be located in its totality within the area defined bythe arms 3 a, 3 b of the angle mirror 13, the joint line of the upperedges 23 and the arms. If again, it is desirable to image and inspectonly two opposite optical axes 9 and a surface having a dimensionperpendicular to the angle bisector 14, i.e. an upper surface 1 a and alower surface 1 b, it will be sufficient that the lower surface iswithin the area defined by the joint line of the upper edges 23 of thearms 3 a, 3 b of the angle mirror 13 or the joint plane J and the arms,because then the lower surface 1 b is imaged via the arms of the anglemirror and the upper surface 1 directly to the telecentric imaging unit.The angle α between the arms 3 a and 3 b of the angle mirror ispreferably 90°, as can be seen in the figures. The object 10 ispreferably located between the arms 3 a and 3 b of the angle mirror in amanner such that it is within the area defined by the angle bisector 14and one of the arms, either arm 3 a or arm 3 b. Then the lower surface 1b is imaged via both the arms of the angle mirror 13 and each of theside surfaces 1 c and 1 d is imaged via only one arm of the anglemirror, i.e. arm 3 a or arm 3 b. When the angle mirror 13 is used, thefirst viewing direction P1 oriented towards the lower surface 1, 1 b ofthe object, i.e. viewed from the imaging unit to the rear side of theobject 10, is formed via both the arms 3 a and 3 b of the angle mirror13, as illustrated in FIGS. 1-9. The second viewing direction P2oriented towards the upper surface 1, 1 a of the object, i.e. to thefront side of the object 10 viewed from the imaging unit, is formedwithout any angle mirror, because it can be directly seen. The third andfourth viewing direction P3, P4 oriented towards the lateral surfaces 1,1 c and 1, 1 d of the object, i.e. to the sides of the object 10 viewedfrom the imaging unit, are both formed via one arm 3 a or 3 b of theangle mirror 13, as can be seen in FIGS. 1-9.

Further in accordance with the invention, the imaging direction D_(P)parallel with the optical axis 9 of the telecentric imaging unit 11, 12has been positioned or oriented with respect to the combination of theobject 10 and the angle mirror 13, and the device comprises auxiliarymirrors 8 within the imaging area K of the telecentric imaging unit,between this and the object. The auxiliary mirrors are marked with thegeneral reference numeral 8, which implies any special type of auxiliarymirror. The specifying reference numerals 8 a-8 i are used only when itis desirable to make a difference between the auxiliary mirrors used atdifferent locations and/or in different positions. These auxiliarymirrors 8 are oriented and placed at such intervals from saidtelecentric imaging unit 11 and/or 12 that differences of imagingdistance of the viewing directions P1 and/or P2 and/or P3 and/or P4 viathe two arms 3 a and 3 b or one arm 3 a or 3 b or not via the arms ofthe angle mirror are compensated as they pass via said auxiliarymirrors. Imaging distances and routes of the viewing directions signifythe distance of travel of a light beam forming the image from thesurface of the object either 1} reflected directly via the auxiliarymirrors to a given location of the telecentric imaging unit or 2}reflected from the surface of the object via one arm of the angle mirrorand via the auxiliary mirrors to the given location of the imaging unitmentioned above or 3} reflected from the surface of the object via thesurfaces of two arms of the angle mirror and possibly via auxiliarymirrors to the given location of the telecentric imaging unit mentionedabove. The difference of imaging distance naturally denotes thedifference between the lengths measured along these different imagingdistances or intervals. Thus, for instance, it can be understood fromFIG. 7 that the imaging distance length m1 from the lower surface 1 b ofthe object reflected via the two arms of the angle mirror is the longestone, the imaging distance length m3 from the first lateral surface 1 dreflected via one arm of the angle mirror is slightly shorter, theimaging distance length m4 from one lateral surface 1 c of the objectreflected via the second arm of the angle mirror is still shorter andthe imaging distance length m2 from the upper surface 1 a of the objectdirectly without any angle mirror is the shortest of all, when theimaging distance lengths are measured in the same transverse plane, inthis case the joint plane J of the upper edges 23 of the angle mirror.The differences m1-m2, m1-m3 and m1-m4 between the imaging distanceslengths are compensated with auxiliary mirrors and their locationspacings. Depending on the mutual position of the camera and theparabolic mirror 24 of the telecentric imaging unit and of the generaldirection of the telecentric imaging unit relative to the combination ofangle mirror and object, the auxiliary mirrors can be disposed eitherindividually or in couples along imaging distances corresponding tospecific predetermined viewing directions P1 and/or P2 and/or P3 and/orP4, so that the image-generating light beams are reflected via them. Theexact positions of the auxiliary mirrors depend on each application, anexpert being capable of fixing them on the basis of the data provided inthis text and the figures by general optical principles, so that theywill not be explained in detail here. It is hence particularly essentialto use an angle mirror 13 showing the rear side and the sides of theobject 10, and auxiliary mirrors 8, which compensate for the differencesin the imaging distance lengths above. The concave parabolic mirror orthe planar parabolic mirror 24 and said auxiliary mirrors 8, 8 a-8 i, 18are strip mirrors, whose reflective surfaces are transverse to theoptical axis.

FIG. 7 illustrates one of the most straightforward embodiments of theinvention, in which the concave planar parabolic mirror 24 faces towardsthe objective 22 and away from the combination of object and anglemirror. In this case, the device comprises at least a number ofauxiliary mirrors 8 equalling the number of desired viewing directionsof the object P1 and/or P2 and/or P3 and/or P4, at least one auxiliarymirror 8 g, 8 c, 8 a, 8 e being disposed for each viewing direction toreflect information from the object 10 to the telecentric imaging unit11. Explained in detail, the image-generating light beams, i.e. theinformation from the object, follow the following imaging distances byreflection. Viewing direction P1 to the lower surface of the object.From the lower surface 1 b via the two arms 3 b and 3 a of the anglemirror 13 to the auxiliary mirror 8 g, which is located at only a smallinterval h1 from the planar parabolic mirror 24 and thus from thetelecentric imaging unit 11, and via the auxiliary mirror of this to theimaging unit. Viewing direction P3 to the lateral surface of the object.From the lateral surface 1 d via one arm 3 a of the angle mirror 13 tothe auxiliary mirror 8 a, which is located at a greater interval h3 fromthe planar parabolic mirror 24 and thus from the telecentric imagingunit 11, and via the auxiliary mirror of this to the imaging unit.Viewing direction P4 to the lateral surface of the object. From thelateral surface 1 c via the other arm 3 b of the angle mirror 13 to theauxiliary mirror 8 e, which is located at a still greater interval h4from the planar parabolic mirror 24 and thus from the telecentricimaging unit 11, and via the auxiliary mirror of this to the imagingunit. Viewing direction P2 to the upper surface of the object. From theupper surface 1 a to the auxiliary mirror 8 c, which is at the greatestinterval h2 from the planar parabolic mirror 24 and thus from thetelecentric imaging unit 11, and via the auxiliary mirror of this to theimaging unit. The intervals h1, h2, h3 and h4 have been selected suchthat 2×h1+m1=2×h2+m2=2×h3+m3=2×h4+m4, the telecentric imaging unitviewing all the sides of the object as located at the same interval.

FIG. 8 illustrates another embodiment of the invention, in which theconcave planar parabolic mirror 24 faces towards the objective 22 andthe combination of object and angle mirror. The device then comprises atleast one pair of auxiliary mirrors 8 a and 8 b; 8 c and 8 d; 8 e and 8f disposed for each desired viewing direction P2 and/or P3 and/or P4,which is shorter than the viewing direction P1 and/or P3 and/or P4having the longest imaging distance, each pair of auxiliary mirrorsbeing disposed to reflect information from the object 10 to thetelecentric imaging unit. Explained in detail, the image-generatinglight beams, i.e. the information from the object, passes along thefollowing imaging distances by reflection. Viewing direction P1 to thelower surface of the object. From the lower surface 1 b via the two arms3 b and 3 a of the angle mirror 13 directly to the telecentric imagingunit 11. Viewing direction P3 to the lateral surface of the object. Fromthe lateral surface 1 d via one arm 3 a of the angle mirror 13 to thepair of auxiliary mirrors 8 b and 8 a—in this order—whose mutualinterval is h5, and via this pair of auxiliary mirrors to the imagingunit. Viewing direction P4 to the lateral surface of the object. Fromthe lateral surface 1 c via the second arm 3 b of the angle mirror 13 tothe pair of auxiliary mirrors 8 f and 8 e—in this order—whose mutualinterval h6 is greater, and via this pair of auxiliary mirrors to theimaging unit. Viewing direction P2 to the upper surface of the object.From the upper surface 1 a to the pair of auxiliary mirrors 8 d and 8c—in this order—whose mutual interval h7 is the greatest, via this pairof auxiliary mirrors to the imaging unit. The mutual intervals betweenthe pairs of auxiliary mirrors h5, h6 and h7 have been selected suchthat m1=2×h5+m2=2×h6+m3=2×h7+m4, the telecentric imaging unit viewingall the sides of the object as being at the same interval.

FIGS. 1 and 2 are general views of a preferred embodiment of theinvention, in which a concave planar parabolic mirror 24 faces in thesame direction as the objective 22 and towards the combination of objectand angle mirror. For the sake of clarity, the information paths andmirror positions corresponding to the four viewing directions P1, P2, P3and P4 of this embodiment are shown separately. Firstly, the devicecomprises an auxiliary mirror 18 in common for all the viewingdirections placed between the planar parabolic mirror and the objectivein order to reflect information from the planar parabolic mirror to saidcamera 21. In addition, the device comprises at least one pair ofauxiliary mirrors 8 a and 8 b; 8 e and 8 b; 8 d and 8 c disposed foreach desired viewing direction P2 and/or P3 and/or P4, which is shorterthan the viewing direction P1 and/or P3 and/or P4 having the longestimaging distance, each pair of auxiliary mirrors being disposed toreflect information from the object 10 to the telecentric imaging unit.This embodiment may additionally comprise at least one additional pairof auxiliary mirrors 8 h and 8 i to lengthen the imaging distancecorresponding to one or more viewing directions P1 and/or P2 and/or P3and/or P4 by means of to-and-fro reflection between these. In theembodiment of the figures, these additional auxiliary mirrors 8 h, 8 iare located on a imaging distance corresponding to the second viewingdirection P2, as can be seen in FIGS. 4A and 4B. Generally speaking, theadditional pairs of auxiliary mirrors can be placed between theauxiliary mirrors and the information coming from the object, or betweenthe auxiliary mirrors and the telecentric imaging unit, or between theauxiliary mirrors of the pairs of auxiliary mirrors, or between thepairs of auxiliary mirrors and the information from the object, orbetween the pairs of auxiliary mirrors and the telecentric imaging unit.Explained in detail, the image-generating light beams, i.e. theinformation from the object, passes along the following imagingdistances by reflection. Viewing direction P1 to the lower surface ofthe object: From the lower surface 1 b via the two arms 3 b and 3 a ofthe angle mirror 13 directly to the telecentric imaging unit 11, whichconsequently includes the common auxiliary mirror 18. Viewing directionP3 to the lateral surface of the object: From the lateral surface 1 dvia one arm 3 a of the angle mirror 13 to the pair of auxiliary mirrors8 b and 8 a—in this order—which have a mutual interval, and via thispair of auxiliary mirrors to the imaging unit. Imaging direction P4 tothe lateral surface of the object: From the lateral surface 1 c via onearm 3 b of the angle mirror 13 to the pair of auxiliary mirrors 8 b and8 e—in this order—whose mutual interval is greater, and via this pair ofauxiliary mirrors to the imaging unit. Viewing direction P2 to the uppersurface of the object: From the upper surface 1 a first to the pair ofauxiliary mirrors 8 d and 8 c—in this order—and from there to the pairof auxiliary mirrors 8 h and 8 i—in this order, which two pairs ofauxiliary mirrors in total have the greatest mutual interval, and viathese pairs of auxiliary mirrors to the imaging unit. The mutualintervals between the pairs of auxiliary mirrors and their positionshave been selected such that the intervals shown in the figures meet thefollowing conditions:(/_(1.1)+/_(1.2))+/_(1.3)+/_(1.4)+/_(1.5)+/_(1.6)=(/_(2.1)+/_(2.2))+/_(2.3)+2×/_(2.4)+/_(2.5)+/_(2.6)=/_(3.1)+_(3.2)+/_(3.3)+/_(3.4)+/_(3.5)+/_(3.6)=/_(4.1)+/_(4.2)+/_(4.3)+/_(4.4)+/_(4.5)+/_(4.6),and then the telecentric imaging unit views all the sides of the objectas being at the same interval.

The presentation above allows the conclusion that the imaging distancesof the telecentric imaging unit 11 and 12 from the lower surface 1 b,upper surface 1 a and lateral surfaces 1 c, 1 d of the object to thetelecentric imaging unit 11 have been disposed to be equal in thefollowing manners. For the first viewing direction P1 from the lowersurface 1, 1 b of the object, this has been done by leaving out theauxiliary mirrors or by means of small mutual intervals between theauxiliary mirrors or pairs of auxiliary mirrors. For the second viewingdirection P2 from the upper surface 1, 1 a of the object, this has beendone by means of great intervals between the auxiliary mirrors or pairsof auxiliary mirrors. For the third and fourth viewing directions P3, P4from the lateral surfaces 1, 1 c, 1 d of the object, this has been doneby means of mutual intervals between the auxiliary mirrors or pairs ofauxiliary mirrors. We point out in this context that the invention isapplicable both to inspection of the object 10 from all four viewingdirections P1-P4 also to inspection of the object from three or only twodifferent viewing directions, which three or two viewing directions maybe any one of the four viewing directions mentioned above. Thus, ifdesired, the object can be inspected only with respect to its uppersurface 1, 1 a and to its lower surface 1, 1 b. The choice of surface ofthe object to be inspected naturally depends on the nature of the objectand its purpose of use, which are naturally selected by the user, i.e.which he knows and decides in advance. The device of the invention canbe devised and constructed with all the four viewing directionsavailable, although it has been decided in advance that some of them arenot used, or in some cases a device of the invention can be devised andconstructed so as to comprise means for implementing only two or threeviewing directions P1 and/or P2 and/or P3 and/or P4. In this case, thedesign of the device will be simplified to the extent corresponding tothe auxiliary mirrors or pairs of auxiliary mirrors corresponding to theexcluded direction or directions.

The image plane 23 formed by the light-sensitive pixels of the camera 21has dimensions for receiving at least two partial images I1 and/or I2and/or I3 and/or I4, one of the partial images corresponding to one ofthe directions P1, P2, P3, P4 for inspecting the object. In other words,in accordance with the invention, all the viewing directions usedgenerate simultaneously an image of the object on the image plane 23 ofthe camera or cameras. To this end, the image plane 23 formed by thelight-sensitive pixels of the camera 21 has dimensions for receivingfour partial images, each of which corresponds to one of the viewingdirections P1, P2, P3, P4. The image plane 23 formed by thelight-sensitive pixels of the camera 21 may have one dimension, i.e.length, the camera being a linear camera, and in that case the partialimages I1 and/or I2 and/or I3 and/or I4 are disposed by means ofauxiliary mirrors and/or pairs of auxiliary mirrors in alignment asillustrated in FIG. 10. Since the image plane 23 may optionally have adimension in two mutually perpendicular directions, i.e. length andwidth, in the case of a surface camera, the partial images I1 and/or I2and/or I3 and/or I4 can be disposed by means of auxiliary mirrors and/orpairs of auxiliary mirrors either exactly aligned or slightlyoverlapping as shown in FIG. 11.

The device of the invention may comprise also a second telecentricimaging unit 12, which comprises a non-telecentric camera 21, whichconsists of an objective 22 and an image plane 23 formed oflight-sensitive pixels, and a concave planar parabolic mirror 24, withthe objective aperture 25 located in the focal plane F of this. Hencethis second imaging unit 12 is of the same type as the first imagingunit 11. Each of the first and second telecentric imaging unit receivesan image from at least two viewing directions P1 and/or P2 and/or P3and/or P4 following the principles explained above in this description.FIG. 9 illustrates such a case of two telecentric imaging units 11 and12. Regarding the array of auxiliary mirrors 8, the embodiment of FIG. 9is on principle of the same type as the embodiment of FIG. 8, with onlyimages of the lower surface and one lateral surface of the object beingreflected to the first imaging unit 11, while the images of the uppersurface and the second lateral surface are reflected to the secondimaging unit 12. However, in this case, the differences between theimaging distance lengths caused by the angle mirror as explained abovehave to be compensated only per one single telecentric imaging unit. Inother words, only two imaging distance lengths m1 and m3 need to becompensated with respect to each other and two other imaging distancelengths m2 and m4 need to be compensated only with respect to eachother. We also point out with respect to this embodiment that, in thiscase as well, the telecentric imaging units 11, 12 are directed towardsthe combination of object and angle mirror, but with the orientationperformed by reflection via the auxiliary mirrors 8 a, 8 g, 8 c and 8 ein an inclined position. The angle bisector 14 of the angle mirror 13and the optical axes 9 of the telecentric imaging units 11, 12, moreprecisely their extensions in this case, are also aligned, and the anglebisector 14 and the optical axes join, because the two imaging unitshave been turned by and angle and the auxiliary mirrors turn all theoptical features by exactly the same angle.

While there have been shown and described and pointed out fundamentalnovel features of the invention as applied to preferred embodimentsthereof, it will be understood that various omissions and substitutionsand changes in the form and details of the devices and methods describedmay be made by those skilled in the art without departing from thespirit of the invention. For example, it is expressly intended that allcombinations of those elements and/or method steps which performsubstantially the same function in substantially the same way to achievethe same results are within the scope of the invention. Moreover, itshould be recognized that structures and/or elements and/or method stepsshown and/or described in connection with any disclosed form orembodiment of the invention may be incorporated in any other disclosedor described or suggested form or embodiment as a general matter ofdesign choice. It is the intention, therefore, to be limited only asindicated by the scope of the claims appended hereto. Furthermore, inthe claims means-plus-function clauses are intended to cover thestructures described herein as performing the recited function and notonly structural equivalents, but also equivalent structures. Thusalthough a nail and a screw may not be structural equivalents in that anail employs a cylindrical surface to secure wooden parts together,whereas a screw employs a helical surface, in the environment offastening wooden parts, a nail and a screw may be equivalent structures.

1. A device for optical inspection of open surfaces of an object from atleast two different viewing directions, the device comprising: anillumination source for illuminating the open surfaces of the object, asensor for detecting the light intensity reflected by differentlocations of the open surfaces of the object and for converting it intoelectric form, the sensor in the device comprising at least a firsttelecentric imaging unit having an optical axis; and an angle mirror andauxiliary mirrors within an imaging area of the telecentric imagingunit, between the telecentric imaging unit and the object; the deviceconfigured so that the object is positioned between the arms of theangle mirror the telecentric imaging unit is directed towards acombination of the object and the angle mirror, and said auxiliarymirrors are oriented and placed at intervals from said telecentricimaging unit that differences of imaging distances of light beamsbetween the viewing directions from the surfaces of the object to thetelecentric imaging unit are compensated with respect to imagingdistance lengths as the light beams pass via said auxiliary mirrors. 2.A device as defined in claim 1, wherein said illumination source arediffused light sources or directional light sources, which emitincoherent radiation.
 3. A device as defined in claim 1, wherein theangle bisector of the angle mirror and said optical axis or theirextensions are aligned; and that said angle bisector and said opticalaxis join.
 4. A device as defined in claim 3, wherein the concaveplane-parabolic mirror faces towards the objective and away from thecombination of the object and the angle mirror; and that the devicecomprises at least such number of auxiliary mirrors equalling the numberof desired viewing directions towards the object, whereupon at least oneauxiliary mirror is disposed for each viewing direction to reflectinformation from the object to the telecentric imaging unit.
 5. A deviceas defined in claim 4, wherein it further comprises at least oneadditional pair of auxiliary mirrors to lengthen the imaging distancecorresponding to one or more viewing directions by means of to-and-froreflection between these auxiliary mirrors; and that the additionalpair(s) of auxiliary mirrors are placed: between the auxiliary mirrorsand the information coming from the object, or between the auxiliarymirrors and the telecentric imaging unit, or between the auxiliarymirrors of the pairs of auxiliary mirrors, or between the pairs ofauxiliary mirrors and the information coming from the object, or betweenthe pairs of auxiliary mirrors and the telecentric imaging unit.
 6. Adevice as defined in claim 3, wherein the concave plane-parabolic mirrorfaces towards the objective and towards the combination of the objectand the angle mirror; that the device comprises at least one pair ofauxiliary mirrors disposed for each such desired viewing direction thatis shorter than the viewing direction having the longest imagingdistance; and that each pair of auxiliary mirrors is disposed to reflectinformation from the object to the telecentric imaging unit.
 7. A deviceas defined in claim 3, wherein the concave plane-parabolic mirror facesin the same direction as the objective and towards the combination ofthe object and the angle mirror; that the device comprises an auxiliarymirror in common for all of the viewing directions located between theplane-parabolic mirror and the objective to reflect information from theplane-parabolic mirror to said camera, and at least one pair ofauxiliary mirrors is disposed for each such desired viewing directionthat is shorter than the viewing direction having the longest imagingdistance; and that each pair of auxiliary mirrors is disposed to reflectinformation from the object to the telecentric imaging unit.
 8. A deviceas defined in claim 1, wherein the first telecentric imaging unitcomprises: a non-telecentric camera having an objective and an imageplane formed of light-sensitive pixels, and a concave plane-parabolicmirror, with an aperture of the objective located in the focal plane ofsaid mirror.
 9. A device as defined in claim 1, wherein said concaveplane-parabolic mirror and said auxiliary mirrors are strip mirrors,whose reflective surfaces are transverse to the optical axis and whichare disposed each in a different manner so laterally off the opticalaxis that all of the viewing directions to a lower surface, uppersurface and lateral surfaces of the object are simultaneously availablewithout mutual shading between the auxiliary mirrors.
 10. A device asdefined in claim 1, wherein the imaging distances from a lower surface,upper surface and lateral surfaces of the object to the telecentricimaging unit have been disposed with equal lengths: for a first viewingdirection from the lower surface of the object without auxiliary mirrorsor with small mutual intervals between the auxiliary mirrors or pairs ofauxiliary mirrors, for a second viewing direction from the upper surfaceof the object with long mutual intervals between the auxiliary mirrorsor pairs of auxiliary mirrors, for a third and a fourth viewingdirection from the lateral surfaces of the object with medium mutualintervals between the auxiliary mirrors or pairs of auxiliary mirrors.11. A device as defined in claim 1, wherein the object is disposedwithin the area defined by an angle bisector and one arm of the anglemirror.
 12. A device as defined in claim 1, wherein the angle betweenthe arms of the angle mirror is 90°.
 13. A device as defined in claim 1,wherein the image plane formed by the light-sensitive pixels of thecamera has a length and/or width for receiving at least two partialimages, each of which corresponds to one of the viewing directions. 14.A device as defined in claim 1 further comprising a second telecentricimaging unit comprising: a non-telecentric camera having an objectiveand an image plane formed of light-sensitive pixels, and a concaveplane-parabolic mirror, with an aperture of the objective located in thefocal plane of this device, and that both the first and the secondtelecentric imaging unit receive an image at least from two viewingdirections.
 15. A device as defined in claim 1, wherein saidcompensation reduces said differences in imaging distance lengths tozero.
 16. A device for optical inspection of open surfaces of an objectfrom at least two different viewing directions, the device comprising:means for illuminating the open surfaces of the object, means fordetecting the light intensity reflected by different locations of theopen surfaces of the object and for converting it into electric form,said means for detecting comprising at least a first telecentric imagingunit having an optical axis; and an angle mirror and auxiliary mirrorswithin an imaging area of the telecentric imaging unit, between thetelecentric imaging unit and the object; the device configured so thatthe object is positioned between the arms of the angle mirror, thetelecentric imaging unit is directed towards a combination of the objectand the angle mirror, and said auxiliary mirrors are oriented and placedat such intervals from said telecentric imaging unit that differences ofimaging distances of light beams between the viewing directions from thesurfaces of the object to the telecentric imaging unit are compensatedwith respect to imaging distance lengths as the light beams pass viasaid auxiliary mirrors.
 17. A device as defined in claim 16, whereinsaid means for illuminating are diffused light sources or directionallight sources, which emit incoherent radiation.
 18. a device as definedin claim 16, wherein said compensation reduces said differences inimaging distance lengths to zero.